1. Field of the Invention
This invention relates to batteries and more particularly to apparatus and methods for improving the performance of lithium-sulfur batteries.
2. Description of the Related Art
Our society has come to rely on batteries to power a myriad of devices, including computers, cell phones, portable music players, lighting devices, as well as many other electronic components. Nevertheless, there is an ongoing need for further advances in battery technology. For example, there is still a significant need for economical batteries that can power automobiles or provide load-leveling capabilities for wind, solar, or other energy technologies. Furthermore, the “information age” increasingly demands portable energy sources that provide lighter weight, higher energy, longer discharge times, more “cycles”, and smaller customized designs. To achieve these advances, technologists continue to work to develop batteries with higher and higher energy densities while still providing acceptable safety, power densities, cost, and other needed characteristics.
Lithium-sulfur (Li—S) batteries offer great potential to meet many of the above-stated needs. The theoretical specific energy of lithium-sulfur batteries is 2600 Wh/kg, which is one of the highest known energy densities for batteries that use non-gaseous constituents. The materials needed to produce these batteries are light, energetic, inexpensive, and readily available. In contrast with most cathode materials, sulfur is relatively non-toxic, making these batteries relatively safe for human contact.
Nevertheless, rechargeable lithium-sulfur batteries have failed to achieve commercial success for several reasons. These reasons include: (1) rapid capacity fade on cycling; (2) high self-discharge; and (3) poor utilization of the cathode. The first two reasons, namely capacity fade on cycling and high self-discharge, are related. These problems primarily occur because some of the cathode constituents, namely lithium polysulfides, are soluble in typical electrolytes. When a porous or microporous separator is used, these cathode constituents tend to migrate to the anode with each cycle, resulting in irreversible capacity loss. Although some researchers have used polymer backbones or binders in the cathode to immobilize polysulfides and thereby improve cycle stability, the stability is undesirably accompanied by poor cathode utilization and hence disappointing specific energy.
One prior art attempt to resolve some of the above-stated problems is disclosed in U.S. Pat. No. 6,852,450 issued to Hwang et al. (hereinafter “Hwang”), which is herein incorporated by reference. In this reference, Hwang attempts to improve cathode utilization by recognizing the differences in dissolution characteristics between elemental sulfur, and lithium sulfide or lithium polysulfide. Hwang teaches that sulfur is apolar and dissolves best in an apolar solvent such as benzene, fluorobenzene, toluene, trifluortoluene, xylene, cyclohexane, tetrahydrofurane, or 2-methyl tetrahydrofurane. Lithium sulfide and polysulfides are polar and thus are best dissolved in polar solvents such as a carbonate organic solvent or tetraglyme. In addition, an effective electronic conductor, such as SUPER P Li™ Conductive Carbon Black (hereinafter “Super P carbon”), may be added to the cathode constituents to improve electrical conductivity.
In one example, Hwang used the solvents tetrahydrofurane/propylene carbonate/dimethyl carbonate in a 20/40/40 ratio in the cathode. The third solvent was intentionally selected to be a relatively viscous solvent to reduce the impact of constituent migration through the micro-porous membrane in the Hwang battery. The cathode initially consisted of sixty percent elemental sulfur with twenty percent Super P carbon, and twenty percent polyvinyl acetate (PVA). The latter constituent was apparently added to reduce the mobility of the soluble species and to serve as a binder. By using an apolar and polar solvent mixture to partially dissolve both elemental sulfur and lithium sulfides and polysulfides. Hwang was able to achieve impressive specific capacities when cycling between 1.5V and 2.8V at various C-rates. Hwang was initially able to demonstrate about 1000 Wh/kg specific energy while cycling at a 1C rate. However, capacity was lost with each subsequent cycle.
In view of the foregoing, what is needed is a lithium-sulfur battery that equals or improves upon the cathode utilization achieved by Hwang, while also reducing the capacity fade and self-discharge exhibited by the Hwang battery.